This invention relates to a magnetic recording medium, more specifically to a magnetic recording medium which is improved in the dispersion properties and the orientation properties of magnetic particles in a magnetic coating film suitable for a high density recording.
A usual magnetic recording medium can be obtained by first dispersing a magnetic powder together with a binder resin so as to prepare a paint-like composition, applying the prepared composition to the surface of a support such as a polyester film, subjecting it to an orientation treatment, drying it and finally passing it through a surface smoothing process.
Such recording media are widely employed for a recording in VCR's audio systems and the like, but the increase in the density of the recording media is now required from the viewpoint of the demand of a prolonged recording time. As recording systems, there are no longitudinal magnetic recording systems in which a recording is carried out by the use of the recording medium magnetized inside it and along a lengthwise direction thereof, and a perpendicular recording system in which a recording is carried out by the use of the recording medium magnetized in a perpendicular direction thereof. The latter system is particularly suitable for the high density recording, and the researches on this perpendicular recording medium have extensively been conducted in recent years (see, for example, "Perpendicular Orientation of Magnetic Powder", K. Sumiya et al., IEEE transaction on magnetics, MAG-20, January 19; Proceedings of "Symposium on Magnetic Media Manufacturing Method", MMS-C-1 (1983), D. E. Speliotis & L. B. Lueck published by magnetic media Information services; "Tushin Gakkai Giho", MR 80-28, 17 (1980), Y. Sato et al.; and IEEE transaction on magnetics, MAG-18, 1122 (1982), O. Kubo et al.).
As the magnetic powder for use in the above-mentioned perpendicular recording medium, much attention is paid to a hexagonal system powder. Each hexagonal magnetic particle has a hexagonal plate shape and possesses magnetization easy axes. Thus, when the magnetic powder is applied to the surface of the support, the magnetization easy axes will easily be oriented perpendicularly to the support surface, which fact is advantageous.
In order to obtain the high density recording medium by the use of the hexagonal system magnetic powder, the size of each magnetic particle is preferably less than a recording minimum wavelength or a bit length. Further, the magnetic particles are required to be uniformly dispersed in the magnetic recording medium.
These requirements are based on the ground that if the magnetic particles in the recording medium are coagulated and if their dispersion is insufficient, surface properties and the like of the medium will be lowered, so that the deterioration in its reproduction output and an increase in noise will result, thereby obtaining no medium having a high performance.
However, each magnetic particle having such a minimum size takes a single magnetic domain structure. That is to say, it constitutes a minute magnet. Therefore, the magnetic particles tend to be magnetically bound with each other, and if bound, they will be difficult to be uniformly dispersed into the binder resin and their orientation properties will also be lowered.